I dont think we'll see it.
As I understand it, TNG would have to be completely re-edited to make HD versions as they (and Ds9 and VOY) were edited on video. Thats assuming they kept all the original film stock. Also, every special effect would have to be redone. Every phaser blast, every transporter effect. It would take much, much longer than TOS. Where they often only had 10-15 fx shots per show, TNG had a lot more
It worked out cheaper at the time, but in hindsight it may age the show even further